The present invention relates to an antilock brake control device capable of braking the wheels of an automobile at a maximum braking efficiency.
In order to control the brake assembly in an automobile efficiently according to the change in the friction with the road, an antilock brake control device should have an antilock control function which selectively reduces, holds and increases the braking pressure, while the brake pedal is being trodden, to repeatedly release and apply the brake at very short time intervals so as to increase the braking efficiency to its maximum.
A control unit for such an antilock control comprises wheel speed sensors for detecting the wheel rotations, an electronic control circuit for calculating the wheel speeds, decelerations, estimated vehicle speed and the slip rates of the wheels from the detected wheel rotation signals to produce control signals for increasing, holding or reducing the braking pressure on the basis of the results of the calculations, and a fluid pressure control unit which receives the control signals from the control circuit and adjusts the braking pressure from a master cylinder to feed them to respective wheel cylinders.
The electronic control circuit performs various operations as described above from the detected wheel rotation signals to judge whether the wheels are showing a tendency to lock or recover from the locked state by checking the fluctuations of the slip rates and decelerations. On the basis of the result of the judgement, it supplies control signals for reducing, holding or increasing the braking pressure to the fluid pressure control unit.
An ordinary fluid control unit of this type comprises solenoid valves (cutoff valves and flow control valves may be added optionally), check valves, hydraulic pumps with motors, accumulators and reservoir tanks. The valves of one of the above types are provided in the braking lines between the master cylinder and the wheel cylinders to control the flow of braking pressure or pump pressure.
There are three known methods for controlling the flow from the master cylinder to the wheel cylinders to brake the wheels, namely the four-channel control in which one set of solenoid valves are allocated to each wheel to control the wheels independently of one another, the three-channel control in which one set of solenoid valves are allocated to each of the right and left front wheels and another set is allocated to both of the rear wheels, and the two-channel control in which one set of solenoid valves are allocated to each of the front wheels so that the braking pressures on the rear wheels will follow the pressure on one of the front wheels.
As a method of applying fluid pressure control signals to the hydraulic circuit, it is known to reduce the braking pressure on both the front wheels or both the rear wheels at one time if one of the front wheels or one of the rear wheels at the side put under lower hydraulic pressure (namely the side where the coefficient of friction with the road surface is smaller) shows a tendency to lock (such a control mode is hereinafter referred to as the select-low mode). Another known method is to reduce the braking pressure on the wheels at both sides if the wheel at one side put under higher hydraulic pressure shows a tendency to lock (select-high mode). Still another method is known as the independent control mode in which the respective wheels are controlled independently of one another according to the road condition. It is also known to reduce the braking pressure on each diagonally opposed pair of wheels associated by X piping, if one of the pair of wheels rotating at a lower speed shows a tendency to lock (diagonal select-low mode).
The select-low mode is known to be effective in increasing the resistance of the wheels to lateral forces. This will lead to an improvement in the directional stability and drivability of the vehicle. But with this method, it is often difficult to gain a sufficient braking force. Thus, the braking distance tends to be longer. In the select-high mode, the wheels can be braked with a sufficient braking force but the resistance to lateral forces are not enough. The independent control mode tends to be costly but has an advantage that the wheels can be controlled delicately according to the road condition. In view of advantages and disadvantages of the above-described control modes, it has been believed to be the best way to control the front wheels on the independent or select-high principle and control the rear wheels on the select-low principle.
Such prior art methods are intended for front wheel drive (FWD) cars and rear wheel drive (RWD) cars, but can also be applied to four wheel drive (4WD) cars by adding some modifications to adapt to their driving system as disclosed e.g. in Japanese Patent Publication 62-238158. This publication discloses an antilock control method applicable to a car capable of switching its driving mode between FWD and 4WD. When the driving mode switches, the antilock control mode, too, is adapted to switch from one mode to another mode. Namely, during the FWD mode, both front wheels are controlled independently of each other and both rear wheels are controlled on the select-low mode (three-channel control). During the 4WD mode, each diagonally opposed pair of wheels are controlled on the select-low mode (two-channel control).
A 4WD car has a front shaft and a rear shaft joined together through a viscous coupling. If there is a difference in rotating speed between the front and rear shafts, the driving force on the wheels connected to one of the front and rear shafts rotating at a lower speed might not be efficiently transmitted to the road surface. This problem is prevented because such a driving force is partially transmitted to the other shaft through the viscous coupling, thus increasing the driving force on the other wheels. While the abovementioned difference in the rotating speed is small, the torque transmitted between the front and rear shafts through the viscous coupling is small enough not to cause interference between axles. Thus, the wheels can be braked with a sufficient braking force by controlling the respective wheels independently of one another.
But if the difference in rotating speed rises above a certain level, the torque transmitted between the shafts and thus the interference between the axles will increase to such an extent as to cause jerkiness or skidding of the vehicle and thus to worsen its directional stability.
In the above noted prior art, this problem is tackled by controlling the wheels on the diagonal select-low mode. With this control mode, the torque transmitted between the shafts can be reduced to a minimum, because the respective diagonally opposed pairs of wheels are controlled on the select-low basis and thus the difference of torque between the front and rear wheels can be kept small while keeping high directional stability of the vehicle.
But, because the viscous coupling has a limited torque transmission capability, if the wheels are controlled on the diagonal select-low principle and especially if the coefficient of friction at one side differs from that at the other side, the braking distance might be extended. Furthermore, if the vehicle is running on a road having an extremely low coefficient of friction, the slip rates of the wheels tend to increase even if they are controlled on the diagonal select-low principle, thus lowering the directional stability of the vehicle.
In the aforementioned prior art methods, the wheels were always controlled on the diagonal select-low principle during the 4WD mode in spite of those problems.